It has three parameters, votage control of gain, a selection of clipping diodes and fine vintage sound.

The name 308 refers to the LM308 single op-amp which is used in this circuit.

The 308 offers three parameters on the faceplate: TONE sets the brightness level, FILTER dials in the high frequency response, and DRIVE determines the signal gain. Latter parameter can be voltage controlled.

A small DIP switch on the faceplate selects which diodes are used for clipping: 1N4148 diodes, LEDs, germanium diodes or none at all.
Input impedance can be set with a trimm potentiometer.